My U.S. Pat. No. 4,259,968 discloses a flow head for a spirometer having a breath transmission passage with opposite ends open to the atmosphere. A patient breathes through the breath transmission passage bi-directionally. An inside tube having a substantially smaller diameter than the breath transmission passage produces a restriction on air flow between the ends of the passage. Intermediate ports through the wall of the breath transmission passage are located near the restriction, i.e., near the ends of the inside tube where gas velocity is low and thus turbulence is small. Flexible tubing connections to the ports on the breath transmission passage lead to a flow rate sensor controlled by a source of bias gas. The sensor measures a pressure drop in the flow head produced by the bi-directional gas flow from a patient through the breath transmission passage. An external bridge circuit becomes unbalanced in response to the bi-directional flow of gas through the passage and this produces a measurement of gas flow.
In the flow head described in the '968 patent, the patient breathes through flexible tubing coupled to the upstream end of the flow head. Such patient gas delivery tubing can be an endotracheal tube adapted for releasable connection to the end of the breath transmission passage. There are different sizes of patient gas flow tubes depending upon patient breath flow rate. An endotracheal tube for pediatric use, for example, is much smaller in diameter than adult patient sizes. Such a gas flow head is typically used with a gas flow rate sensor using a supply of bias gas to produce accurate flow measurements. This flow head is not easily adaptable to use with a pressure sensor.
Flowmeters are used to measure patient gas volume in spirometry, in ventilators, or in metabolic studies where the amount of calories given to a patient can be determined if an accurate flowmeter is available. A challenging problem is to extend the range of flowmeter accuracy in the low flow area, such as in pediatric uses. Flowmeters could also be used for making critical gas flow measurements in anesthesia.
The flow head should be placed next to the patient's endotracheal tube in order to measure bi-directional gas flow at the patient. Most flowmeters are placed at the exhalation side and, therefore, they do not produce inhalation flow measurements. By knowing both inhalation and exhalation flows, metabolic studies can be performed and the amount of calories (effort) expended by the patient can be calculated. In addition, gas leakage can be calculated.
For pediatric use a patient gas delivery tube of 15 mm diameter or less is commonly used; a 22 mm gas delivery tube is commonly used for adult anesthesia and spirometry. For pediatric use there can be a large amount of dead volume in the flow head. This dead volume should be minimized to inhibit the baby from rebreathing exhaled breath. In one recently introduced monitor the dead volume is 1.4 cc. For an infant who breathes only a 5 cc volume, 1.4 cc is a large volume to rebreathe.
The present invention is based on a recognition that significant improvements in the accuracy of the output from the flow head sensor can be produced by designing the structure of the flow head to match certain sizes of patient breath delivery tubes. Flowmeters accurate in the low flow range for pediatric use can be produced. The invention also results in a less expensive flow head; it minimizes dead volume; and it can be used with a pressure sensor coupled to the flow head for measuring gas flow, which in many instances can be an advantage over a sensor that detects flow and requires a source of bias gas.